Speed up build under MSVC

I noticed the MSVC build getting slow, so I added a couple of things to the precompiled header, which sped up the build noticeably. Here's a couple of representative samples from my VM, using `msbuild /ds /m /t:libgit2_clar libgit2.sln` on a solution generated with `cmake -G "Visual Studio 11 Win64"`:

* Before: `Time Elapsed 00:03:14.73`
* After: `Time Elapsed 00:00:57.19`